My understanding is that its very well documented, but the funding isn't there to do much with it. I mean most of the team behind things like the Atari computers, Acorn, Commodore, early Apples, etc are retired but we can emulate them and understand these things on a very technical level. I know this isn't a great analogy, but ultimately age of the project or the age of the original team members isn't the bottleneck.
I sometimes see retro projects on hack-a-day done by people who could be the grandchildren of the original designers of those vintage chips and OS's. They probably know more about that chip or OS than the original people do just due to them being out of the game for so long. The same way people regularly lose to fifth graders in tests because they dont recall 5th grade science and civics. If anything your scenario might be the opposite! Grandpa might be asking his granddaughter how those registers worked or how to emulate his OS from 1982.
I remember reading about the team that maintains the Voyagers. Its a skeleton crew using legacy equipment to keep the communication going with the assumption the next decade, or even earlier, is going to be it.
NASA has the same problem the private sector has. Capitalism rewards things that will generate profit/prestige, not legacy cost centers, and NASA is not immune from that dynamic as NASA employees and managers want to maximize their income and prestige too. So the people maintaining or bug fixing old products are often lowest on the prestige, pay, recruiting, and profit spectrum than those chasing new things. They get the skeleton crew funding and can't do novel things, even if technically possible because of lack of staff and buy-in.
Passion projects make for feel-good documentaries like 'It's Quieter in the Twilight' but ultimately if society isn't vested in these teams, their hands will always be tied.